The present invention relates generally to a family of phosphodiesterases designated PDE8A and uses thereof.
Phosphodiesterases (PDEs) hydrolyze 3xe2x80x2,5xe2x80x2 cyclic nucleotides to their respective nucleoside 5xe2x80x2 monophosphates. The cyclic nucleotides cAMP and cGMP are synthesized by adenylyl and guanylyl cyclases, respectively, and serve as second messengers in a number of cellular signaling pathways. The duration and strength of the second messenger signal is a function of the rate of synthesis and the rate of hydrolysis of the cyclic nucleotide.
Multiple families of PDEs have been identified. The nomenclature system includes first a number that indicates the PDE family. To date, seven families (PDE1-7) are known which are classified by: (i) primary structure; (ii) substrate preference; (iii) response to different modulators; (iv) sensitivity to specific inhibitors; and (v) modes of regulation [Loughney and Ferguson, in Phosphodiesterase Inhibitors, Schudt, et al. (Eds.), Academic Press: New York, New York (1996) pp. 1-19]. The number indicating the family is followed by a capital letter, indicating a distinct gene, and the capital letter followed by a second number, indicating a specific splice variant or a specific transcript which utilizes a unique transcription initiation site.
The amino acid sequences of all mammalian PDEs identified to date include a highly conserved region of approximately 270 amino acids located in the carboxy terminal half of the protein [Charbonneau, et al., Proc. Natl. Acad Sci. (USA) 83:9308-9312 (1986)]. The conserved domain includes the catalytic site for cAMP and/or cGMP hydrolysis and two putative zinc binding sites as well as family specific determinants [Beavo, Physiol. Rev. 75:725-748 (1995); Francis, el al., J. Biol. Chem. 269:22477-22480 (1994)]. The amino terminal regions of the various PDEs are highly variable and include other family specific determinants such as: (i) calmodulin binding sites (PDE1); (ii) noncatalytic cyclic GMP binding sites (PDE2, PDE5, PDE6); (iii) membrane targeting sites (PDE4); (iv) hydrophobic membrane association sites (PDE3); and (v) phosphorylation sites for either the calmodulin-dependent kinase II (PDE1), the cAMP-dependent kinase (PDE1, PDE3, PDE4), or the cGMP dependent kinase (PDE5) [Beavo, Physiol. Rev. 75:725-748 (1995); Manganiello, et al., Arch. Biochem. Acta 322:1-13 (1995); Conti, et al., Physiol. Rev. 75:723-748 (1995)].
Members of the PDE1 family are activated by calcium-calmodulin. Three genes have been identified; PDE1A and PDE1B preferentially hydrolyze cGMP while PDE1C has been shown to exhibit a high affinity for both cAMP and cGMP. The PDE2 family is characterized as being specifically stimulated by cGMP [Loughney and Ferguson, supra]. Only one gene has been identified, PDE2A, the enzyme product of which is specifically inhibited by erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA). Enzymes in the PDE3 family are specifically inhibited by cGMP. Two genes are known, PDE3A and PDE3B, both having high affinity for both cAMP and cGMP, although the Vmax for cGMP hydrolysis is low enough that cGMP functions as a competitive inhibitor for cAMP hydrolysis. PDE3 enzymes are specifically inhibited by milinone and enoximone [Loughney and Ferguson, supra]. The PDE4 family effects cAMP hydrolysis and includes four genes, PDE4A, PDE4B, PDE4C, and PDE4D, each having multiple splice variants. Members of this family are specifically inhibited by the anti-depressant drug rolipram. Members of PDE5 family bind cGMP at non-catalytic sites and preferentially hydrolyze cGMP. Only one gene, PDE5A, has been identified. The photoreceptor PDE6 enzymes specifically hydrolyze cGMP [oughney and Ferguson, supra]. Genes include PDE6A and PDE6B (the protein products of which dimerize and bind two copies of a smaller xcex3 inhibitory subunit to form rod PDE), in addition to PDE6C which associates with three smaller proteins to form cone PDE. The PDE7 family effects cAMP hydrolysis but, in contrast to the PDE4 family, is not inhibited by rolipram [Loughney and Ferguson, supra]. Only one gene, PDE7A, has been identified.
1. Given the importance of cAMP and cGMP in intracellular second messenger signaling, there thus exists an ongoing need in the art to identify addition PDE species. Identification of heretofore unknown families of PDEs, and genes and splice variants thereof, will provide additional pharmacological approaches to treating conditions in which cyclic nucleotide pathways are aberrant as well as conditions in which modulation of intracellular cAMP and/or cGMP levels in certain cell types is desirable.
In brief, the present invention provides polypeptides and underlying polynucleotides for a novel PDE family designated PDE8. The invention includes both naturally occurring and non-naturally occurring PDE8 polynucleotides and polypeptide products thereof Naturally occurring PDE8 products include distinct gene and polypeptide species within the PDE8 family (i.e., PDE8A); these species include those which are expressed within cells of the same animal and well as corresponding species homologs expressed in cells of other animals. Within each PDE8 species, the invention further provides splice variants encoded by the same polynucleotide but which arise from distinct mRNA transcripts (i.e., PDE8A1 and PDE8A2). Non-naturally occurring PDE8 products include variants of the naturally occurring products such as analogs (i.e., wherein one or more amino acids are added, substituted, or deleted) and those PDE8 products which include covalent modifications (i.e., fusion proteins, glycosylation variants, Metxe2x88x921PDE8s, Metxe2x88x922-Lysxe2x88x921-PDE8s, Glyxe2x88x921PDE8s and the like). The PDE8 family is distinguished from previously known PDE families in exhibiting high affinity for hydrolysis of both cAMP and cGMP but relatively low sensitivity to enzyme inhibitors specific for other PDE families. In a preferred embodiment, the invention provides a polynucleotide comprising the sequence set forth in SEQ ID NO: 1. The invention also embraces polynucleotides encoding the amino acid sequence set out in SEQ ID NO: 2. A presently preferred polypeptide of the invention comprises the amino acid sequence set out in SEQ ID NO: 2. The invention provides two splice variant cDNAs which give rise to two polypeptides designated PDE8A1 and PDE8A2. PDE8A1 and PDE8A2 polypeptides, and the polynucleotides encoding the polypeptides, are discussed herein as representative of the PDE8 enzyme family embraced by the invention.
The present invention provides novel purified and isolated polynucleotides (e.g., DNA sequences and RNA transcripts, both sense and complementary antisense strands, including splice variants thereof) encoding the human PDE8s. DNA sequences of the invention include genomic and cDNA sequences as well as wholly or partially chemically synthesized DNA sequences. xe2x80x9cSynthesized,xe2x80x9d as used herein and is understood in the art, refers to purely chemical, as opposed to enzymatic, methods for producing polynucleotides. xe2x80x9cWhollyxe2x80x9d synthesized DNA sequences are therefore produced entirely by chemical means, and xe2x80x9cpartiallyxe2x80x9d synthesized DNAs embrace those wherein only portions of the resulting DNA were produced by chemical means. A preferred DNA sequence encoding a human PDE8 polypeptide is set out in SEQ ID NO: 1. Also preferred are polynucleotides encoding the PBE8 polypeptide of SEQ ID NO: 2 and the PDE8A1 and PDE8A2 splice variant polypeptides set out in SEQ ID NOs: 6 and 4, respectively. Preferred polynucleotides encoding PDE8A1 and PDE8A2 are set out in SEQ ID NOs: 5 and 3, respectively. The invention further embraces species, preferably mammalian, homologs of the human PDE8 DNA.
The invention also embraces DNA sequences encoding PDE8 species which hybridize under moderately stringent conditions to the non-coding strands, or complements, of the polynucleotides in SEQ ID NOs: 1, 3 and 5. DNA sequences encoding PDE8A polypeptides which would hybridize thereto but for the redundancy of the genetic code are contemplated by the invention. Exemplary moderate hybridization conditions are as follows: hybridization at 65xc2x0 C. in 3xc3x97SSC, 0.1% sarkosyl, and 20 mM sodium phosphate, pH 6.8, and washing at 65xc2x0 C. in 2xc3x97SSC with 0.1% SDS. It is understood in the art that conditions of equivalent stringency can be achieved through variation of temperature and buffer, or salt concentration as described Ausebel, et al. (Eds.), Protocols in Molecular Biology, John Wiley and Sons (1994), pp. 6.0.3 to 6.4.10. Modifications in hybridization conditions can be empirically determined or precisely calculated based on the length and the percentage of guanosine/cytosine (GC) base pairing of the probe. The hybridization conditions can be calculated as described in Sambrook, et al., (Eds.), Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory Press: Cold Spring Harbor, N.Y. (1989), pp. 9.47 to 9.51.
Autonomously replicating recombinant expression constructions such as plasmid and viral DNA vectors incorporating PDE8 sequences are also provided. Expression constructs wherein PDE8-encoding polynucleotides are operatively linked to an endogenous or exogenous expression control DNA sequence and a transcription terminator are also provided.
According to another aspect of the invention, host cells are provided, including procaryotic and eukaryotic cells, either stably or transiently transformed with DNA sequences of the invention in a manner which permits expression of PDE8 polypeptides of the invention. Host cells of the invention are a valuable source of immunogen for development of antibodies specifically immunoreactive with PDE8. Host cells of the invention are also conspicuously useful in methods for large scale production of PDE8 polypeptides wherein the cells are grown in a suitable culture medium and the desired polypeptide products are isolated from the cells or from the medium in which the cells are grown by, for example, immunoaffinity purification.
Knowledge of PDE8 DNA sequences allows for modification of cells to permit, or increase, expression of endogenous PDE8. Cells can be modified (e.g., by homologous recombination) to provide increased PDE8 expression by replacing, in whole or in part, the naturally occurring PDE8 promoter with all or part of a heterologous promoter so that the cells express PDE8 at higher levels. The heterologous promoter is inserted in such a manner that it is operatively-linked to PDE8 encoding sequences. See, for example, PCT International Publication No. WO 94/12650, PCT International Publication No. WO 92/20808, and PCT International Publication No. 91/09955. The invention also contemplates that, in addition to heterologous promoter DNA, amplifiable marker DNA (e.g., ada, dhfr, and the multifunctional CAD gene which encodes carbamyl phosphate synthase, aspartate transcarbamylase, and dihydroorotase) and/or intron DNA may be inserted along with the heterologous promoter DNA. If linked to the PDE8 coding sequence, amplification of the marker DNA by standard selection methods results in co-amplification of the PDE8 coding sequences in the cells.
The DNA sequence information provided by the present invention also makes possible the development through, e.g homologous recombination or xe2x80x9cknock-outxe2x80x9d strategies [Capecchi, Science 244:1288-1292 (1989)], of animals that fail to express functional PDE8 or that express a variant of PDE8. Such animals are useful as models for studying the in vivo activities of PDE8 and modulators of PDE8.
The invention also provides purified and isolated mammalian PDE8 polypeptides. Presently preferred PDE8A polypeptides are set out in SEQ ID NOs: 4 and 6. Most preferred is a PDE8 polypeptide comprising the amino acid sequence set out in SEQ ID NO: 2. PDE8 polypeptides of the invention may be isolated from natural cell sources or may be chemically synthesized, but are preferably produced by recombinant procedures involving host cells of the invention. Use of mammalian host cells is expected to provide for such post-translational modifications (e.g., glycosylation, truncation, lipidation, and phosphorylation) as may be needed to confer optimal biological activity on recombinant expression products of the invention. PDE8 products of the invention may be fall length polypeptides, biologically active fragments, or variants thereof which retain specific PDE8 biological activity. Variants may comprise PDE8 polypeptide analogs wherein one or more of the specified (i.e., naturally encoded) amino acids is deleted or replaced or wherein one or more non-specified amino acids are added: (1) without loss of one or more of the biological activities or immunological characteristics specific for PDE8; or (2) with specific disablement of a particular biological activity of PDE8.
Variant products of the invention include mature PDE8A products, i.e., PDE8 products wherein leader or signal sequences are removed, having additional amino terminal residues. PDE8 products having an additional methionine residue at position -1 (Metxe2x88x921-PDE8) are contemplated, as are PDE8 products having additional methionine and lysine residues at positions -2 and -1 (Metxe2x88x922-Lysxe2x88x921-PDE8). Variants of these types are particularly useful for recombinant protein production in bacterial cell types.
The invention also embraces PDE8 variants having additional amino acid residues which result from use of specific expression systems. For example, use of commercially available vectors that express a desired polypeptide such as a glutathione-S-transferase (GST) fusion product provide the desired polypeptide having an additional glycine residue at position -1 as a result of cleavage of the GST component from the desired polypeptide. Variants which result from expression in other vector systems are also contemplated.
The invention further embraces PDE8 products modified to include one or more water soluble polymer attachments. Particularly preferred are PDE8 products covalently modified with polyethylene glycol (PEG) subunits. Water soluble polymers may be bonded at specific positions, for example at the amino terminus of the PDE8 products, or randomly attached to one or more side chains of the polypeptide.
Also comprehended by the present invention are antibodies (e.g., monoclonal and polyclonal antibodies, single chain antibodies, chimeric antibodies, CDR-grafted antibodies and the like) and other binding proteins specific for PDE8 products or fragments thereof. Specific binding proteins can be developed using isolated or recombinant PDE8 products, PDE8 variants, or cells expressing such products. Binding proteins are useful for purifying PDE8 products and detection or quantification of PDE8 products in fluid and tissue samples using known immunological procedures. Binding proteins are also manifestly useful in modulating (i.e., blocking, inhibiting or stimulating) biological activities of PDE8, especially those activities involved in signal transduction. Anti-idiotypic antibodies specific for anti-PDE8 antibodies are also contemplated.
The scientific value of the information contributed through the disclosures of DNA and amino acid sequences of the present invention is manifest. As one series of examples, knowledge of the sequence of a cDNA for PDE8A makes possible through use of Southern hybridization or polymerase chain reaction (PCR) the identification of genomic DNA sequences encoding PDE8 and PDE8 expression control regulatory sequences such as promoters, operators, enhancers, repressors, and the like. DNA/DNA hybridization procedures carried out with DNA sequences of the invention under moderately to highly stringent conditions are likewise expected to allow the isolation of DNAs encoding allelic variants of PDE8A; allelic variants are known in the art to include structurally related proteins sharing one or more of the biochemical and/or immunological properties specific to PDE8A. Similarly, non-human species genes encoding proteins homologous to PDE8A can also be identified by Southern and/or PCR analysis. As an alternative, complementation studies can be useful for identifying other human PDE8 products as well as non-human proteins, and DNAs encoding the proteins, sharing one or more biological properties of PDE8A.
Polynucleotides of the invention are also useful in hybridization assays to detect the capacity of cells to express PDE8. Polynucleotides of the invention may also be the basis for diagnostic methods useful for identifying a genetic alteration(s) in a PDE8 locus that underlies a disease state or states.
Also made available by the invention are anti-sense polynucleotides which recognize and hybridize to polynucleotides encoding PDE8. Full length and fragment anti-sense polynucleotides are provided. Anti-sense polynucleotides are particularly relevant to regulating expression of PDE8 by those cells expressing PDE8 mRNA.
The DNA and amino acid sequence information provided by the present invention also makes possible the systematic analysis of the structure and function of PDE8s. DNA and amino acid sequence information for PDE8 also permits identification of molecules with which PDE8A will interact. Agents that modulate (i.e., increase, decrease, or block) PDE8 activity may be identified by incubating a putative modulator with PDE8 and determining the effect of the putative modulator on PDE8 phosphodiesterase activity. The selectivity of a compound that modulates the activity of the PDE8 can be evaluated by comparing its activity on the PDE8 to its activity on other PDE enzymes. Cell based methods, such as di-hybrid assays and split hybrid assays, as well as in vitro methods, including assays wherein a polypeptide or its binding partner are immobilized, and solution assays are contemplated by the invention.
Selective modulators may include, for example, antibodies and other proteins or peptides which specifically bind to the PDE8 or PDE8 nucleic acid, oligonucleotides which specifically bind to the PDE8 or PDE8 nucleic acid, and other non-peptide compounds (e.g., isolated or synthetic organic molecules) which specifically react with PDE8 or PDE8-encoding nucleic acid. Mutant forms of PDE8 which affect the enzymatic activity or cellular localization of the wild-type PDE8 are also contemplated by the invention. Presently preferred targets for the development of selective modulators include, for example: (1) regions of the PDE8 which contact other proteins and/or localize the PDE8 within a cell, (2) regions of the PDE8 which bind substrate, (3) allosteric cyclic nucleotide-binding site(s) of PDE8, (4) phosphorylation site(s) of PDE8 and (5) regions of the PDE8 which are involved in multimerization of PDE8 subunits. Modulators of PDE8 activity may be therapeutically useful in treatment of a wide range of diseases and physiological conditions in which PDE activity is known to be involved.
The invention further contemplates small molecule modulators of PDE8A enzyme activity. There are at least three different types of libraries used for the identification of small molecule modulators. These include: (1) chemical libraries, (2) natural product libraries, and (3) combinatorial libraries comprised of random peptides, oligonucleotides or organic molecules.
Chemical libraries consist of structural analogs of known compounds or compounds that are identified as xe2x80x9chitsxe2x80x9d or xe2x80x9cleadsxe2x80x9d via natural product screening. Natural product libraries are collections of microorganisms, animals, plants, or marine organisms which are used to create mixtures for screening by: (1) fermentation and extraction of broths from soil, plant or marine microorganisms or (2) extraction of plants or marine organisms. Combinatorial libraries are composed of large numbers of peptides, oligonucleotides or organic compounds as a mixture. They are relatively easy to prepare by traditional automated synthesis methods, PCR, cloning or proprietary synthetic methods. Of particular interest are peptide and oligonucleotide combinatorial libraries. Still other libraries of interest include peptide, protein, peptidomimetic, multiparallel synthetic collection, recombinatorial, and polypeptide libraries. For a review of combinatorial chemistry and libraries created therefrom, see Myers, Curr. Opion. Biotechnol. 8:701-707 (1997).
Identification of modulators through use of the various libraries described herein permits modification of the candidate xe2x80x9chitxe2x80x9d (or xe2x80x9cleadxe2x80x9d) to optimize the capacity of the xe2x80x9chitxe2x80x9d to modulate activity.
The invention further provides methods to identify a specific binding partner compound of a PDE8A polypeptide of the invention comprising the steps of: a) contacting the PDE8A polypeptide with a compound under conditions which permit binding between the compound and the PDE8A polypeptide; b) detecting binding of the compound to the PDE8A polypeptide; and c) identifying the compound as a specific binding partner of the PDE8A polypeptide. Binding partner identified in the methods of the invention preferably modulate PDE8A enzyme activity, either through inhibition or activation, or enhancement, of the enzyme.
The invention also provides methods to identify a specific binding partner compound of a PDE8A polynucleotide of the invention comprising the steps of: a) contacting the PDE8A polynucleotide with a compound under conditions which permit binding between the compound and the PDE8A polynucleotide; b) detecting binding of the compound to the PDE8A polynucleotide; and c) identifying the compound as a specific binding partner of the PDE8A polynucleotide. The binding partner of the PDE8A polynucleotide preferably modulates expression of the PDE8A polypeptide encoded by the PDE8A polynucleotide, either through inhibiting expression or enhancing expression.
The invention also provides compounds identified by a method of the invention, as well as compositions comprising a compound identified and a pharmaceutically acceptable carrier.